This invention is directed to a thermoplastic composition which is conductive to ions. More particularly, the composition comprises a polymeric adduct and alkali metal salt. The adduct has an acrylic backbone and polyether side chains.
Polymeric solid electrolytes exhibiting high ionic conductivities are excellent materials for commercial devices such as batteries, electrochromic displays, sensors, and capacitors. Liquid electrolytes have conventionally been used in such devices. When conventional liquid electrolytes are employed, e.g., in electrochromic devices, there is a possibility of leakage of the electrolyte and the inherent difficulty of sealing such a device. Thus, although liquid materials perform the required ionic transport function, they are very difficult to handle from a device construction standpoint.
Various solid electrolytes have been suggested for use in electrochromic devices. Solid electrolytes of complex halides are known, particularly iodides of silver with an alkali metal. Additionally, solid electrolytes may comprise aluminum compounds such as sodium beta-alumina and potassium beta-alumina. However, these electrolytes are all typically expensive to prepare and, in the case of the alumina compounds, could not be formed directly on components of an electrochromic device since they require very high processing temperatures. Others have suggested forming a solid electrolyte comprising a sheet of porous glass impregnated with a solid, ion-conductive silver or alkali metal compound. One disadvantage of employing such an impregnated glass sheet is that, because it is a solid of limited flexibility, it would be difficult to assemble the component layers of an electrochromic device and achieve the intimate contact required between this sheet and the adjacent layers.
Such problems are minimized with polymeric electrolytes which are generally either extremely viscous liquids or tacky solids. It would be desirable, however, if the polymeric electrolytes were solids which could be formed into self-supporting films with a degree of flexibility to allow uncomplicated device fabrication. Such solid polymeric electrolyte formulations, if used in an automotive windshield comprising an electrochromic, device might allow compliance with automotive requirements regarding occupant restraint and glass shatter.
One approach to forming such a flexible solid material involves copolymerization of polyethylene oxide and polypropylene oxide and results in systems having good conductivity at room temperature but a markedly reduced ionic conductivity at low temperatures. Another approach to the formation of room temperature flexible solid polymer electrolytes includes forming materials having backbones of phosphazene and siloxane-based polymers with etheric side chains. These materials generally lack appreciable dimensional stability and must be chemically or radiationally cross-linked. In addition, the susceptibility of the Si--O--C bonds to hydrolysis and subsequent structural degradation constitute a severe problem unless moisture can entirely be eliminated.
Yet another approach involves poly (2-methoxy polyethylene glycol monomethacrylates) complexed with a lithium salt of triflic acid having etheric side chains of 9-22 ethylene oxide units. Others have reported similar polymers coplymerized with styrene. Common problems encountered by each of these is the inverse relationship between optimization of the ionic conductivity and simultaneously the mechanical properties. The dependence of ionic conductivity on salt concentration in amorphous materials must also be optimized. Decreasing the salt content, while it desirably reduces crystallinity, also reduces the mechanical strength of the resulting material. In these and similar systems, purification and neutralization steps necessary in processing such materials is very time consuming.
The present invention provides a thermoplastic ionic conductor that overcomes the deficiencies of the foregoing materials in that it shows no high temperature instability regardless of water content, reacts rapidly without phase separation or exclusion of a plasticizer when it is used, and forms tough flexible films via normal casting or extrusion techniques. It is a thermoplastic material able to perform well in an electrochromic-laminated glass system as would be necessary, particularly in automotive applications.